Make _merge_isolated_script_shards robust to harness failures.

scripts/slave/recipes/chromium.expected/dynamic_swarmed_failed_isolated_script_test.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.linux",
       "--builder",
       "Linux Tests",
@@ skipping 401 matching lines @@
       "/path/to/tmp/"
     ],
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "telemetry_gpu_unittests on Ubuntu-12.04",
     "~followup_annotations": [
       "step returned non-zero exit code: 255",
       "@@@STEP_TEXT@<br/>failures:<br/>test1.Test1<br/>test2.Test2<br/>@@@",
       "@@@STEP_LOG_LINE@outdir_json@{@@@",
-      "@@@STEP_LOG_LINE@outdir_json@  \"0/output.json\": \"{\\\"successes\\\": [], \\\"failures\\\": [\\\"test1.Test1\\\", \\\"test2.Test2\\\"], \\\"valid\\\": true}\", @@@",
+      "@@@STEP_LOG_LINE@outdir_json@  \"0/output.json\": \"{\\\"successes\\\": [], \\\"failures\\\": [\\\"test1.Test1\\\", \\\"test2.Test2\\\"], \\\"valid\\\": true, \\\"times\\\": {\\\"test2.Test2\\\": 0.1, \\\"test1.Test1\\\": 0.1}}\", @@@",
       "@@@STEP_LOG_LINE@outdir_json@  \"summary.json\": \"{\\\"shards\\\": [{\\\"internal_failure\\\": false, \\\"failure\\\": false}]}\"@@@",
       "@@@STEP_LOG_LINE@outdir_json@}@@@",
       "@@@STEP_LOG_END@outdir_json@@@",
       "@@@STEP_LINK@shard #0@https://chromium-swarm.appspot.com/user/task/10000@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 tests failed: ['telemetry_gpu_unittests']",
     "recipe_result": null,
     "status_code": 1
   }
 ]